A capacitance type or piezo resistor type pressure sensor is conventionally known as a semiconductor pressure sensor. Among these sensors, one with a piezo resistor element formed on a silicon substrate can be integrated together with a peripheral circuit, etc., into a single chip and easily manufactured, and is therefore used in various application fields and has an extensive range of pressures to be measured.
This type of semiconductor pressure sensor is provided with a diaphragm formed through etching of a semiconductor substrate and a piezo resistor element formed with a predetermined positional relationship with the diaphragm.
Japanese Patent Application Laid-Open No. 2000-214022 discloses a semiconductor pressure sensor with the position of a piezo resistor element from an end of a diaphragm set according to the thickness of the diaphragm. According to the document, a diaphragm having a width H1 of 580 μm parallel to the <001> direction and a width H2 of 630 μm parallel to the <−110> direction perpendicular thereto is formed on a single crystal silicon (110) substrate. In that case, when the piezo resistor element is placed along the width H2 direction, the relationship between the thickness of the diaphragm and the peak position of compression stress from the center of the diaphragm is as shown in FIG. 21. In FIG. 21, when the thickness of the diaphragm is smaller than 60 μm, the peak position of compression stress is in the vicinity of the end of the diaphragm. When the thickness of the diaphragm is greater than 60 μm, the peak position of compression stress is outside the diaphragm and the distance increases as the thickness of the diaphragm increases.
“Optimal Design of Piezo Resistance Pressure Sensor” (Masayuki Yoneda, [online], Aug. 1, 2000, Yamatake Corporation, Internet <URL: http://jp.yamatake.com/corp/rp/tech/review/pdf/2000—8—01/2000—8—01.pdf> discloses that the output has been substantially doubled using the direction perpendicular to the diaphragm and the direction parallel thereto as current directions. The principal plane of an Si substrate is (100) and four n-type resistors are arranged in a p-type diffusion layer in the <110> direction. FIG. 22 shows a graph of a stress distribution with respect to the distance from the center of the diaphragm in the semiconductor pressure sensor disclosed in the above described URL. The graph shows that when the pressure on the diaphragm decreases, the stress is concentrated on a diaphragm edge.
There are proposals on further miniaturization of a pressure sensor and application for measurements in a living body or for micromachines, etc., in recent years. This requires a high sensitivity pressure sensor having a diaphragm with a smaller area than the actual diaphragm using a semiconductor process which allows integration on a single chip. A resistance variation of silicon is generally determined by a value proportional to the product of a piezo resistance coefficient by stress. In the case of a square diaphragm having a length per side of h and a thickness of a, a maximum value of stress of the diaphragm is proportional to (h/a)2 and sensitivity of the pressure sensor depends on the maximum value of the stress. Therefore, when the size of the diaphragm is reduced, the sensitivity decreases drastically. On the other hand, when the thickness a is decreased so as to increase the maximum value of the stress, the mechanical strength of the diaphragm decreases. Therefore, realizing a smaller pressure sensor requires a structure capable of attaining high sensitivity without reducing the thickness of the diaphragm.
However, the above described documents make no mention of local values as a small pressure sensor of the diaphragm. That is, the above described Japanese Patent Laid-Open Publication discloses the sensor having a distance from the center to the edge of the diaphragm of approximately 300 μm (length of one side is approximately 600 μm). Furthermore, the disclosure at the above described URL shows a technical opinion on a large diaphragm having approximately 440 μm (length of one side is approximately 880 μm). No knowledge is available of stress or resistor arrangement in the vicinity of edges of a pressure sensor including a small diaphragm having a distance from the center to the edge of the diaphragm of 200 μm or less (length of one side is approximately 400 μm).
The above described demand for attaining high sensitivity without reducing the thickness of the diaphragm is not limited to a pressure sensor but common to semiconductor devices including a diaphragm.
Thus, it is an object of the present invention to provide a high sensitivity semiconductor device provided with a small diaphragm, particularly in such a small structure that the distance from the center to the edge is 200 μm or less yet preventing the thickness of the diaphragm from being reduced more than necessary.